Sky and Telescope - April 2015 - 23

NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA

approach to Vesta, Dawn lost one of its four reaction
wheels. These devices permit the spacecraft to maintain
its orientation in space, to keep its solar panels facing the
Sun, its dish antenna pointed at Earth, and its instruments aimed at science targets. Because reaction wheels
are spun using electricity generated by the solar panels,
pointing with reaction wheels is essentially without cost
to the mission - unlike the spacecraft's rockets, which
have a limited supply of hydrazine to power them.
After the loss of the ﬁrst reaction wheel, the mission
scrambled to conserve hydrazine and preserve the three
remaining reaction wheels by using them sparingly. They
completed the Vesta mission with three wheels, but a
second one ground to a halt as they prepared to depart
Vesta in August 2012. Dawn could travel to Ceres without
reaction wheels, but it would be impossible to complete
the Ceres mission as planned without them: there was
simply not enough hydrazine to accomplish all of the
required turns.
After herculean eﬀort, the mission's earthly planners
succeeded in ﬁnding a way for Dawn to complete its Ceres
to-do list with no reaction wheels at all. They had to make
some compromises: Dawn will not turn to talk to Earth
as frequently as it did at Vesta, and will image Ceres
only 9 times as it approaches, compared to the 23 imaging sessions it performed on approach to Vesta. But the
spacecraft will actually gather more science data at Ceres
than in the original plan. Simply by being more patient
and waiting a little longer to gather observations, mission
planners will be able to acquire everything they promised
to at Ceres, including full-color global maps and detailed
gravity data.

VESTA'S BELT Deep grooves wrap around the asteroid Vesta's
circumference, likely created by the reverberation from the impact
that excavated the Rheasilvia basin at the south pole.

Ceres and Vesta Fast Facts
Parameter
Mean Radius (km)

Ceres

Vesta

476

263

9.39 x 1020

2.59 x 1020

Another Kind of Water World

Mass (kg)

Ceres is a very diﬀerent object from Vesta; we know that
already, even without visiting it. A key insight into Ceres
is its density. Vesta has a bulk density nearly identical to
that of silicate rock. Ceres is 80% larger but only twothirds as dense as Vesta. Yet Ceres is too large for it to
have much porosity (unlike smaller asteroids, which can
have lots of void space, resulting in low density). There is
only one plausible explanation: Ceres contains a substantial amount of water ice.
Given the mass and dimensions of Ceres, and assuming initial proportions of elements like those found in
meteorites, geophysicists Tom McCord (now at Bear Fight
Institute) and Christophe Sotin (University of Nantes,
France) simulated how Ceres would have evolved over
time, from a newly condensed mixture of materials to the
modern day, 4.6 billion years later. They found that Ceres
almost certainly diﬀerentiated, its materials separating into a rocky core and watery mantle. Depending on
how much of its water is incorporated into the crystal
structures of its rocky minerals, its water could compose
anywhere from 17 to 27% of its mass. The upper few

Density (kg/m3)

2,075

3,455

Rotation Period (Earth hours)

9.075

5.342

Semimajor Axis (a.u.)

2.76

2.362

Eccentricity

0.079

0.0895

Inclination (degrees)

10.6

7.14

Mean Albedo

0.10

0.40

kilometers of Ceres might always have been too cold for
diﬀerentiation and so would have remained a mixture of
ice and rock. But it's likely that there was a liquid internal
ocean for some part of its history, and it's possible that
such a liquid layer persists today.
But if this story were true, would there be evidence of
it on the surface? Ceres' surface would always have been
frozen solid, but as the body's primordial heat escaped,
the dwarf planet would have cooled, its ocean slowly freezing solid from the top down. A primitive exterior could
hide an evolved interior. On the other hand, any amount
Sk yandTelescope.com April 2015

http://www.SkyandTelescope.com

Sky and Telescope - April 2015

Table of Contents for the Digital Edition of Sky and Telescope - April 2015